Chemical Reactions: Double Replacement

(MOLECULES BREAKING UP FOR ATOMS TO EXCHANGE PARTNERS)

Teacher Reference

Standards:
  NSES Content Standard B, grades 5-8 page 149.

Objectives:
1. To react chemicals in AB + CD --> AD + CB double replacement reactions.
2. To draw reactant and product atoms and molecules in chemical reactions.
3. To separate salts into positive and negative ions.
4. To predict and identify precipitates in solutions.
5. To write chemical equations of double replacement reactions.
6. To identify unknown solutions in containers from colors of precipitates.

VOCABULARY:

1.  double replacement reaction: Reactions shown by the general formula
   AB + CD --> AD + CB.

2.  molecules: Substances made up of atoms or groups of atoms that have equal
   positive and negative charges.

3.  chemical bond: Atoms in molecules are bonded due to their opposite charges; the
   strength of charges determines the strength of the chemical bond.

4.  salt: Molecules that easily separate into charged positive and negative particles
   called ions in water.

5.  ion: The general name for a positively or negatively charged atom or group of
   atoms with a charge.

6.  cation: Generally metal ions with a positive charge.

7.  anion: Generally one or more nonmetal ions with a negative charge.

8.  polyatomic ion: More than one atom in a group with a positive or negative charge;
   atoms in the group stay together and do not split up in water.

9.  dissociation: The breaking up of a salt in water into its positive and negative ions.

10. spectator ions: Ions that are present and which move about in a solution
   but which do not take part in a chemical reaction.

11. precipitate: A solid that is observed in liquids in the form of insoluble colored
    flakes or clumps that eventually settle to the bottom of a liquid.

Materials needed:
  a. solution set of Cu(NO₃)₂, NaOH, NaCl, Na₂CO₃, and KI in dropper
    bottles (each 0.5 M concentration).
  b. printer paper
  c. goggles
  d. solution set of Pb(NO₃)₂, NaOH, NaCl, Na₂CO₃, and KI in dropper
    bottles (each 0.5 M concentration).
  e. solution set of Pb(NO₃)₂, NaOH, NaCl, Na₂CO₃, and KI in dropper
    bottles (each 0.5 M concentration).
  f. solution set of Cu(NO₃)₂, Pb(NO₃)₂, NaOH, Na₂CO₃, and KI in masked dropper
    bottles (0.5 M). Each bottle is marked with a number from 1- 5.

Answers to Selected Problems

Activity 1: Thinking About a Double Replacement Reaction

1. Can you think of a situation where you have something that is exchanged for
   something else? Answer: Using money in a store.

2. Create other double replacement scenarios like the one above that may occur.

	before		-->	after
scenario	AB	CD	-->	AD	CB
at the bank	student with money	bank with receipt	-->	student with receipt	bank with money
with a ball team	player not playing	ball team with low score	-->	player playing	ball team with high score
at a dance	Jim dances with Jane	Mike dances with Rosa	-->	Jim dances with Rosa	Mike dances with Jane

3. Complete the equation:
   Na¹⁺Cl^1-  +  Ag¹⁺Y^1-  -->   __NaY__ + __AgCl___

   Explain what you did. Answer: The atoms within one molecule exchanged partners
   with the atoms of the opposite charge in the other molecule.

4. Try to complete the equation:
   X²⁺Cl^21-  +  Na¹⁺Z^2- -->  __XZ___ + __2 NaCl___

   Explain what you did. Answer: As before, all atoms within one molecule exchanged
  partners with oppositely charged atoms from the other molecule.

5. What do the words disassociate, disassemble, disarrange have in common?
   Answer: In general, to break up something.

Activity 2: Dissociation of Salts - Using Folding Paper Molecules

1. When picking up a bottle of table salt (NaCl) in water, what do you think is in the
  bottle? Answer: Water, sodium and chloride ions.

2.                 Dissociation of NaCl in Water

3.            Dissociation of NaOH in Water 

4. What are the three ions that are produced when Na₂CO₃ dissociates in water?
  Answer: Two sodium ions and one carbonate ion.

5.          Dissociation of Na₂CO₃ into its Ions in Water 

6.         Dissociation of KI into its Ions in Water 

7. What are the three ions that are produced when Cu(NO₃)₂ dissociates in water?
  Answer: One copper ion and two nitrate ions

8.         Dissociation of Cu(NO₃)₂ into its Ions in Water 
 

9. Imagine going to a mall with friends. Suppose that at the mall and by chance you
  meet an acquaintance that you really like and haven't seen for a while. What do
  your friends and the other person's friends do while you talk and catch up on old
  times? Answer: Probably stand around, or wander off.

Activity 3: Reaction of Cu(NO₃)₂ with NaOH, NaCl, Na₂CO₃, and KI

1. What do you think is the meaning of precipitate? Think of the weather. Answer:
  Something that comes from above and settles to the ground, e.g., snow, rain, etc.

2. What do you think causes the colors of pigments in paints?Answer: Pigments in
  paints are finely divided solids that have color. Precipitates could be pigments.

    Table of Reactions of Cu(NO₃)₂ with NaOH, NaCl, Na₂CO₃, and KI

initial salt solution	Na1+OH1-	Na1+Cl1-	Na21+CO32-	K1+I1-
separate ions with a comma	Na1+ , OH1-	Na1+ , Cl1-	2Na1+ , CO32-	K1+ , I1-
Cu2+ precipitate yes or no	yes	no	yes	yes
if yes, color of precipitate	blue		light blue	black; later to black if on paper
formula of Cu2+ precipitate	Cu(OH)2		CuCO3	CuI
spectator ions	2(NO3)1-   Na1+	Cu2+  2(NO3)1-  Na1+   Cl1-	2(NO3)1-   Na1+	2(NO3)1-  Na1+

Comments: In the presence of the iodide ion, the copper (2+) ion is reduced to copper
  (1+) ion while the iodide (1-) is oxidized to free iodine (I₂). The copper iodide
  formula can be written as Cu₂I₂ or simply CuI. The CuI precipitate is reported as a
  white to brownish white precipitate. The small amount of free iodine produced as a
  product reacts with the starch in paper to produce a brown precipitate which turns to
  purple with time.

3. Summarize the colors of the precipitates formed by filling in the blanks. Copper
  forms a special precipitate with the iodine ion because copper changes from a (2+)
  ion to a (1+) ion. The formula for the copper iodide is CuI. Did you notice that copper
  chloride is not a precipitate?

                  Data Table: Colors of Precipitates

precipitate	color
Cu(OH)2	blue
CuCO3	light blue
CuI	brown; later to black if on paper

4. Use the information above to show the equations between Cu(NO₃)₂ and NaOH.

 

5. Use the information above for the equations between Cu(NO₃)₂ and Na₂CO₃.

6. Use the information above to complete the equations between Cu(NO₃)₂ and KI.
  Notice that copper iodide (CuI) here is as a result of a special reaction. 

Activity 4: Reaction of Pb(NO₃)₂ with NaOH, NaCl, Na₂CO₃, and KI
1. Lead nitrate Pb²⁺(NO₃)₂ is a neutral molecule. When in water however, it breaks up
  into three ions. List positive and negative ions that are produced by one molecule
  in water. Answer: One Pb²⁺ ion and two nitrate (NO₃)^1- ions.

2. Prove, by looking at the lead nitrate molecule, that the charge on the (NO₃) ion is 1-?
  Explain. Answer: Since the lead ion is positive, each nitrate ions must be charged
  negatively 1-.

3. What molecules in the list of molecules in NaOH, NaCl, Na₂CO₃, and KI have
  polyatomic ions? What is their charge? Answer: OH^1- and CO₃^2-.

   Table of Reactions of Pb(NO₃)₂ with NaOH, NaCl, Na₂CO₃, and KI

initial salt solution	Na1+OH1-	Na1+Cl1-	Na21+CO32-	K1+I1-
separate ions with a comma	Na1+ , OH1-	Na1+ , Cl1-	2Na1+ , CO32-	K1+ , I1-
Pb2+  precipitate yes  or  no	yes	yes	yes	yes
formula of Pb2+ precipitate	Pb(OH)2	PbCl2	PbCO3	PbI2
color of precipitate	white	slightly white	white	yellow
spectator ions	Na1+   (NO3)1-	Na1+   (NO3)1-	Na1+   (NO3)1-	Na1+   (NO3)1-

4. Summarize the colors of the precipitates formed by filling in the blanks.

                  Table of Colors of Precipitates

precipitate	color
Pb(OH)2	white
PbCO3	white
PbCl2	slightly white
PbI2	yellow

5. Use the information above to show the equations between Pb(NO₃)₂ and NaOH. Use
  darkened circles for oxygen and circles with the symbols of atoms inside for molecules. 


6. Use the information above for the equations between Pb(NO₃)₂ and Na₂CO₃.

 

7. Use the information above to show the equations between Pb(NO₃)₂ and NaCl. 


8. Use the information above for the equations between Pb(NO₃)₂ and KI. 

Activity 5: Doing Detective Work - What's in the Mystery Bottles?

1. Someone placed solutions of Pb(NO₃)₂, NaOH, Na₂CO₃, Cu(NO₃)₂ and KI into
  different bottles without writing the names of the solutions on them. What do you
  think could be done to identify the contents of each bottle? Answer: Test all of the
  recommended combinations of pairs of solutions from the bottles to see if a precipitate
  forms. Colors from precipitates can be used to determine the ion pair that is in each
  bottle. The same number on two different bottles identifies the common ion.

2. Describe a plan by which you could find out the contents of each bottle.
  Answer: Same as Question #1.

  Data Table: Precipitates and Colors of Combinations of Unknown Solutions

combination of unknowns	Pb(NO3)2 & KI 1 and 2	Pb(NO3)2 &Na2CO3 1 and 3	Pb(NO3)2 & NaOH 1 and 5
preciptate color	yellow	white	white
precipitate formula	PbI2	PbCO3	Pb(OH)2
combinations of unknowns	KI & Cu(NO3)2 2 and 4	Na2CO3 &Cu(NO3)2 3 and 4	Cu(NO3)2 & NaOH 4 and 5
preciptate color	brown to purple; black on paper	light blue	blue
precipitate formula	CuI	CuCO3	Cu(OH)2

3. Write your conclusions in the table below.

      Table of Unknowns and Contents of the Mystery Bottles.

number	bottle #1	bottle #2	bottle #3	bottle #4	bottle #5
formula of salt	Pb(NO3)2	KI	Na2CO3	Cu(NO3)2	NaOH

Notes for the teacher:
1. In general, heavy metal ions are hazardous. Health and Safety officials indicate
  that the small amounts of lead and copper used in the activities do not pose a
  problem and can safely be disposed in the trash. Students should always wash their
  hands as a general practice and especially if they spill liquids on themselves.

2. Reactions involving copper and iodide ions produce a precipitate which turns blue
  black on paper due to the starch in the paper. Reactions can be done on plastic
  squares or on the plastic boxes to alleviate reactions of iodine with starch in paper.

3. Solution concentration is generally in Molar concentration (M) is calculated as
  moles/liter of solution.

    To prepare Molar solutions, multiply the desired Molar concentration by the
    formula mass of the compound in grams then place the calculated mass in a
    liter bottle with water in it. Stir to dissolve. Fill the bottle to the liter mark with
    water. The above process can also be completed in proportional amounts.

    For example:
      To calculate a fraction of a Molar solution such as 0.2M, take 0.2 times the
       formula mass of the compound in grams, place in a liter bottle as before
       and fill the bottle to the liter mark with water as before. 

(C) "Armchair Chemistry for the Middle Grades 2001"